Many industrial processes and analytical procedures require a flow of fluids, including industrial and analytical gases, which are used as fuels, heat transfer media, and transfer media for particulate source sampling, for example. These fluids must often be provided at controlled rates of flow. Thus, for example, a gaseous fuel such as natural gas is supplied to a gas burner at a precise, controlled volumetric flow rate to ensure stoichiometric operation of the burner. Process gases, including stack effluent gases, often contain reactive materials and contaminants which are the subject of analytical procedures conducted for the purpose of process optimization and pollution control. The analytical procedures used to test for these materials usually include withdrawing a known volume of the gases and then measuring the concentration of reactive materials and contaminants contained in these samples. Samples of stack effluent gas are usually taken by inserting a tubular probe into a stack and then drawing the sample through the probe.
In order to obtain a gas sample that is representative of the effluent stack gases as a whole, the velocity of the gas at the entrance to the sample probe must be identical to the velocity of the gas flow in the region of the stack adjacent the sample probe, a condition referred to as isokinetic sampling. This is especially true where particulate emissions are the subject of the stack testing. If the sample flow rate is higher than the isokinetic rate, the sample will contain a lower concentration of large particles. Conversely, if the sample flow rate is lower than the isokinetic rate, the sample will contain a high concentration of large particles. This is because the motion of particles entrained in the stack effluent gas depends, among other things, on the aerodynamic diameter of the particles and the velocity of the gas medium.
The aerodynamic diameter of a particle is a calculated effective equivalent diameter used to predict motion of particles in fluids and is a function of the actual particulate diameter and the particle density, size, surface contour and shape. Thus, a fibrous material, asbestos for example, may have a relatively high aerodynamic diameter and exhibit motion in a suspending gas different from a material of similar density because of the fibrous nature of the material. When the flow of the gas sample at the probe entrance equals the flow velocity of the stack gases adjacent to the probe isokinetic sampling conditions are said to be achieved. Also, because the stack flow velocity may not remain constant over time the sample probe flow velocity must be changed, in real time, to equal the stack velocity so that a representative sample of gas may be withdrawn through the sample probe.
Before the volume and flow rate of gases can be controlled to desired values they must first be measured. Devices for measuring and controlling the volume flow rate of gas in industrial processes and analytical procedures include mass flow controllers and volumetric flow controllers. Mass flow controllers measure the actual mass of gases flowing through a conduit. The volumetric flow rate of the gas can be estimated by converting the mass flow to volume using well known gas relationships based upon the molecular weight, temperature and pressure of the gas. Since estimating the volumetric flow rate with these types of flow controllers depends on the molecular weight, temperature and pressure, all of which may vary during an analytical procedure, they tend to exhibit inaccuracies in the range of 3-5%.
Volumetric flow controllers measure directly the volume flow rate of gases passing through a conduit, for example. Volumetric flow controllers include differential pressure flow meters and area flow meters. Differential pressure flow meters include venturi meters, which are often called orifice meters. Venturi meters are tubular devices, having a constricted flow region, through which the gas sample flows to provide a measure of the volume flow rate. A pressure drop occurs down stream of the constricted region relative to the pressure up stream of the constriction. The magnitude of the pressure difference is proportional to the volume flow rate of gas flowing through the venturi. The ability to derive accurate flow rate measurements from venturi flow meters depends on rigorous calibration procedures applied to the venturi flow meter. Once properly calibrated these meters provide a quantitative measure of the flow rate by measuring the pressure drop across the venturi and correlating the pressure drop to an empirically determined flow rate. The pressure drop is typically measured by a differential pressure cell or by a liquid manometer. A desired constant gas flow rate is maintained by maintaining a constant pressure drop across the venturi flow meter.
Area flow controllers are commonly called rotameters. A rotameter is a fluid flow rate controller having a vertically positioned flow tube with a tapered bore. A flow, residing within the tapered bore, rises in the bore as gas is allowed to pass through the meter. The rising float defines an increasing annular gap between the tapered bore and the perimeter of the float. The height to which the float rises gives an indication of the volume flow rate of gas through the bore. The float height is correlated to the volumetric flow rate by calibration procedures. A constant volume flow rate of gas through the rotameter is maintained by maintaining the float at a constant bore height. Rigorous calibration of the rotameter is required to insure accurate flow rate indication.
Both the venturi and rotameter flow controllers are susceptible to contamination of the constricted region, in the case of the venturi meter, and the annular bypass area defined between the rotameter float and bore in the case of the rotameter. These types of volumetric flow meters have inherent errors in the measurement principal resulting in 3 to 5% error of actual flow rates and flow rate control.
Neither the mass flow controllers nor the volumetric flow controllers provide real time quantitative information regarding the total volume of gas delivered. However, it is often necessary to determine the total volume of gas that has been delivered to an industrial process or drawn as a sample in an analytical procedure. For example, as discussed above, stack emissions are often tested for the amount of particulate per unit volume of gas flowing through the stack. Therefore, it is necessary that a known volume of gas be sampled and then analyzed for its particulate content. The above-described volume flow rate meters only provide total volume information indirectly by multiplying the flow rate information by a flow time period. Timing inaccuracies may combine with the inaccuracies present in the above described volumetric flow rate meters such that test results are less than reliable and perhaps useless for some analytical procedures.
Another type of flow meter, a positive displacement meter, provides very accurate measurement of total flow volume. Positive displacement meters include reciprocating diaphragm meters and wet gas meters. One type of reciprocating diaphragm meter is a dry gas meter for metering gases. Reciprocating diaphragm meters comprise a meter chamber separated into opposing metering chambers by a diaphragm mounted for reciprocating movement within the chamber. A shuttle valve assembly alternately places one of the metering chambers in fluid communication with a source of gas at a high pressure introduced through a gas inlet of the meter. The shuttle valve assembly places the opposed metering chamber in fluid communication with a gas outlet of the meter which is at a low pressure relative to the source gas. The high pressure gas causes the diaphragm to travel into the metering chamber maintained at low pressure thereby expelling or exhausting gas from the low pressure metering chamber. The diaphragm reaches the end of its travel within the chamber and then the shuttle valve trips to now place the heretofore low pressure metering chamber in fluid communication with the high pressure gas source and the heretofore high pressure metering chamber in fluid communication with the gas outlet. Therefore the diaphragm now travels in the opposite direction exhausting the gas from the metering chamber. Reciprocating diaphragm meters are often configured with two chambers, each mounting a diaphragm and adapted to reciprocate a predetermined phase angle out of phase. This configuration prevents the shuttle valve from stalling, decreases the dwell time of the shuttle valves and provides a more constant flow of gas rather than a pulsed flow that is typical of single chamber reciprocating diaphragm meters.
In a dry gas meter and other reciprocating diaphragm meters, a fixed volume of gas passing through the meter will produce a fixed movement of a diaphragm mounted in a metering chamber which in turn causes a fixed incremental change in a registration device associated with the meter, such as a system of dials or a counter similar to an automobile odometer. Because dry gas meters are positive displacement devices the incremental change in the registration device is solely a function of the passage of a fixed volume of gas through the device and is not a function of time, per se. The calibration necessary for positive displacement meters is very simple because a known volume of a gas passed through the meter will produce a known displacement of the meter mechanism. Also, dry gas meters tend to be relatively insensitive to temperature and pressure changes of the metered gas.
Positive displacement meters are not adapted for providing real-time flow rate information, however. Volumetric flow rate information is derived by dividing the accumulated volume of gas metered by the positive displacement meter by an extended time interval over which the total volume flowed. Where real time volumetric flow rate information is needed for process control or analytical procedures a calculated flow rate as described above may not be useful.
In some industrial processes and analytical procedures it is necessary to measure and control both the instantaneous gas flow rate and the total gas flow volume. For example, the United States Environmental Protection Agency (EPA) has promulgated various reference methods for measuring specific pollutant emissions from industrial or utility smoke stacks. One of these reference methods is referred to as Reference Method 5 and is widely used for measuring particulate matter emissions from industrial and utility smoke stacks. The design criteria for an instrument suitable for carrying out analytical procedures pursuant to the EPA requirements for Reference Method 5 are codified in the Code of Federal Regulations, 40 C.F.R. .sctn.60, app. A. This regulation requires the use of a dry gas meter for volume flow measurements and a venturi meter for flow rate measurements. The flow rate meters and dry gas meters are used in tandem to measure the rate and volume of gas flow with the required accuracy. But there are problems with this tandem set-up of flow meters. The orifice flow meter requires extensive calibration and is susceptible to inaccuracy because of its non-linearity and its sensitivity to contamination of the constricted region. Thus, although flow measurement and control may be realized with this tandem set up, it does so at the expense of increased cost for instrumentation, calibration and operation. What is needed and what is not available is an apparatus that can function as a gas flow rate meter and a total gas volume meter.